Improve uiSettings cache cleanup mechanism for testing

[gsoldevila]

The following cache self-cleaning mechanism causes multiple tests to emit a warning message, as the logic continues to execute after the tests have finished:

https://github.com/elastic/kibana/blob/18aa549e1fd3ee3a74c154c3e16873bbeb44719e/src/core/server/ui_settings/cache.ts#L30

And the corresponding error:

This affects (at least) the following tests:

• src/core/server/ui_settings/ui_settings_client.test.ts
• src/core/server/elasticsearch/elasticsearch_service.test.ts

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[elasticmachine]

Pinging @elastic/appex-sharedux (Team:SharedUX)